1. Field of the Invention
This invention pertains to navigation and positioning systems. More particularly, this invention pertains to a positioning system incorporating navigational satellites and fixed relays in conjunction with a new calibration and processing system to minimize error and obtain more accurate positional coordinates than is possible under current systems.
2. Description of the Prior Art
Navigation by satellite began with the introduction of the U.S. Navy's TRANSIT system in the mid 1960's which is still in use. This system consists of seven polar-orbiting satellites at low-earth altitudes and can provide 20-50 m accuracy. The spread of the satellite orbits allows only one satellite to pass over a given geographic location at a time. Each satellite transmits a beacon-type signal that is received and converted into positional coordinates by analyzing the Doppler effect exhibited by the received signal plus additional navigational information (i.e., the actual satellite positions) transmitted to the TRANSIT user from another source.
A TRANSIT user typically computes a position fix only after each overhead pass is complete, which nominally occurs once every one and one-half hours. The TRANSIT system consequently cannot provide continuous navigational information and only allows the user to update another system, such as dead-reckoning, that provides continuous information. Technical constraints also make the system unusable for vehicles moving at high-speeds.
The United States Government began implementing the next-generation Global Positioning System (GPS) in the late 1970's to provide continuous updates and to service high speed vehicles. GPS employs satellites in 11,000 mile inclined circular orbits spaced 200 Km, 55.degree. apart and will provide a user with continuous coverage anywhere on earth once the full constellation of satellites is properly placed in orbit. The orbits and operating parameters of GPS satellites are well known to those of ordinary skill in the art and are readily available to the public through various sources. One such reference is the Interavia Space Directory (1990-1991) available from Jane's Information Group and another is the World Satellite Almanac (2d Ed.) authored by Mark Long and published by Howard N. Sams & Co.
Each GPS satellite transmits an encrypted signal for military use and a degraded, unencrypted signal for civilian use. GPS satellites continuously broadcast signals that may be received by anyone with the proper equipment. These carrier signals are superimposed with the respective satellite's ephemeris that must be determined from the content of the GPS signal by the user's software in addition to the three dimensions of position. GPS operates on the principle of multilateration wherein a user determines the intersection of a plurality of range measurements derived from the GPS signals, the range measurements being made simultaneously to separate GPS satellites. The user then ascertains from this intersection his instantaneous latitude, longitude, and altitude.
The range measurements inherently contain an error called an offset bias common to all the measurements created by the unsynchronized operation of the satellite and user clocks. This error will yield an erroneous range measurement, making it appear that the user is either closer to or farther from each of the satellites than is actually the case. These measurements are therefore more accurately termed pseudoranges. Four or more measurements are therefore to ascertain the unknown latitude, longitude, altitude, and offset bias required since the measurements are not truly ranges but are instead pseudoranges obtained from the signals of the respective GPS satellites.
GPS has some serious shortcomings, especially for continuous coverage with five meter accuracy, resulting from several factors. Physical factors include uncertainty in (1) atmospheric propagation delays, (2) precise GPS satellite locations, (which are required to determine a fix), and (3) the accuracy of the timing information that provides the basis of the pseudoranges. Induced errors include intentional degradation by the United States Department of Defense of the unencrypted signal for civilian users, from approximately 25 m accuracy to 100 m accuracy or worse for national security reasons. The best currently available techniques for overcoming this error can only reduce it to about 15 m.
One way of reducing the effects of the error in GPS produced by these factors utilizes differential corrections for the pseudoranges measured by the user to eliminate common errors, namely offset biases. Differential corrections are determined by placing a GPS receiver at a precisely known, fixed reference site, and then measuring the actual errors by comparing the received pseudoranges with the values expected for the known reference site. The differences between the received and expected values are then transmitted to users over a separate communication link to correct their pseudorange measurements before the user's position, i.e., the "fix", is computed.
However, some fix errors are residual and cannot be totally compensated for by differential correction. For example, atmospheric propagation delay errors and satellite
PATENT position errors will vary as the distance from the reference site increases and therefore will not be common to all measurements. A method for reducing the sensitivity of the user fix computation to residual error must also be employed in addition to differential correction.
Sensitivity to residual error depends on the satellite locations with respect to the user's location and the consequent mathematical relationship between basic pseudorange measurement errors and the position computation. This mathematical relationship is called the Position Dilution of Precision (PDOP) and its existence and affect are well known to those of ordinary skill in the art. One treatment of the topic may be found in the publication "Geometric Formulas for Dilution of Precision Calculations", authored by Paul Messett and Karl Rudnick, and published in Vol. 37., No. 4 at page 379 of the Journal of the Institute of Navigation. PDOP is a primarily scaler multiplier that allows the user to estimate fix uncertainty for a given measurement uncertainty. For example, if the measurement error is .+-.10 meters and the PDOP is 3, the user can expect a fix error (i.e., error in calculated position) of .+-.30 meters in a statistical sense.
GPS satellite locations are constantly changing as the satellites move across the sky and so the PDOP also constantly changes regardless of whether the user is stationary. Minimum PDOP occurs when all satellites are uniformly distributed across the sky seen by the user but this does not occur all the time. Users sometimes experience very large PDOP, especially when the satellites become "grouped together", and fix errors correspondingly fluctuate.
There have been previous attempts to reduce PDOP or otherwise improve the accuracy of GPS-based positioning for remote moving users. U.S. Pat. No. 4,812,991 discloses a technique in which assorted GPS measurements are combined in a prescribed manner to reduce the effect of measurement errors. This technique does not reduce the effective PDOP governing the relationship between measurement errors and fix errors or extend the useful coverage of GPS as does the present invention.
U.S. Pat. No. 4,876,550 describes another data processing method which, although not specifically directed at GPS-based positioning, could be used to reduce the effect of nearly-singular geometry (large PDOP) on position fix computations. This method also does not reduce the typical effective PDOP but instead is applicable when the PDOP is so high that it is essentially infinitely large. This is not generally applicable to GPS which is designed to provide reasonable PDOP at all times although the PDOP will still be too large for accurate positioning because of measurement errors.
Alternative geostationary satellite-based systems have been proposed in order to minimize dependence on GPS with its attendant problems. These systems exhibit large latitudinal PDOP because all of the satellites necessarily reside in the earth's equatorial plane. The relatively large PDOP can magnify small errors in height calculation or instrument calibration to cause large latitudinal position errors. The achievable accuracy from these systems does not meet all needs but the coverage they provide is ideal since the satellites are always in view.
Some approaches have also attempted to use privately developed shore-based navigation systems. These systems can be very accurate but are affected by atmospheric variations, even to the extent that some longer range systems are unstable at night. Also, the construction and maintenance of shore based stations may be logistically or politically unfeasible in many parts of the world.
For some critical applications such as public safety and energy exploration, large increases in PDOP are intolerable. Occurrence of PDOP variations caused by changing satellite position can be reduced, as can PDOP itself, by integrating GPS measurements with those made to one or more commercial geostationary communication satellites. Since these geostationary satellites are earth-stationary, their effective PDOPs are virtually constant. Thus, when measurements from geostationary satellites are combined with those from GPS, the overall PDOP is "smoothed out" and large latitudinal errors inherent in geostationary satellite systems can be eliminated.
A system is described in United Kingdom Patent No. 2,180,426 that combines GPS with a geostationary communication satellite to provide a combined navigation/communication capability. A GPS-like signal is transmitted from the communication satellite, presumably for navigational purposes. However, the requirement to synchronize the clock of this signal to the GPS clock is virtually impossible to realize because of circuit delays and other transponder limitations found in commercial communication satellites. No description of how the combined positioning would be accomplished is provided even if satisfactory timing could be realized (e.g., how the location of the communication satellite derived). The concept is unworkable as described.
An intrinsic calibration problem arises when combining GPS and geostationary-based measurement. Unlike GPS where the carrier frequencies are all the same, geostationary satellite signals pass through different receiver and transponder circuits which necessarily create different time delays thereby introducing instrumental biases. These instrumental biases are difficult to determine for a moving user because each received signal has a different frequency requiring separate receiving and processing channels so that switching channels is impractical.
This calibration problem renders the usual software solution for determining common measurement delays applied to GPS signals ineffective. Although instrumental biases can be minimized through instrumentation calibration, they may change at any time afterwards due to environmental factors and dynamic effects for moving users. A new method must be found to eliminate instrumental biases in geostationary satellite measurements in realtime.
It is therefore a feature of this invention that it reduces PDOP variations in obtaining positional coordinates.
It is a further feature of this invention that it minimizes the affect of PDOP in obtaining measurements.
It is a still further feature of this invention that it both compensates for unequal instrument delays and determines and eliminates residual error in the measured data in real time.
It is another feature of this invention that it detects and eliminates instrument biases and residual errors using the entire measurement data set.
It is still another feature of this invention that it employs a unique statistically oriented method for detecting and eliminating error in the data set.
It is a further feature of this invention that it integrates navigational satellite data with geostationary satellite data to obtain more accurate positional coordinates.
It is yet another feature of this invention that it provides positional coordinates that are more accurate than can be obtained from conventional techniques.
It is a further feature of this invention that it relies primarily on existing satellite and telecommunication hardware to reduce the cost of implementing the system.